imageToFloatAndMask

Transfer and ARGB AWT image into a FloatProcessor with its grey values
and a FloatProcessor with its alpha values as [0...1].

Note, this method currently relies on how ImageJ reuses the
pixels of an AWT image as generated by Loader.getFlatAWTImage(...)
for creating a ColorProcessor. This may change in the future as have
many things in the past. This method is then the place to fix it.